For the purpose of controlling the energy supply to the downstream electric motor, the switching device incorporates a control unit and a first current path. A first phase of the supply network is fed to the downstream electric motor via the first current path. The first current path incorporates a first electro-mechanical switch and, connected in series with the first switch, a circuit of a second electro-mechanical switch in parallel with a semiconductor switch. The control unit can output a switching signal for the first switch, for the second switch and for the semiconductor switch, and by these can control the desired switching state of the switches. The first and second switches are, in particular, normally open contact switches which, when a switching signal is applied from the control unit, are held in the closed state. If the switching signal is switched off for the first or second switch then, due to a restoring force (e.g. a spring force which works on a contact of the switch concerned) the switch automatically adopts the open state. Switches of this type are, in particular, relays. When a switching signal is applied by the control unit, the semiconductor switch preferably adopts the electrically conducting state.
The switching device incorporates a supply connection, through which the control unit can draw the energy for the switching signals. Generally, a local supply source is linked to the supply connection of the switching device by way of a wire, so that when the switching device is actively operational a supply voltage is present through the supply connection. Through this supply connection, the switching device draws the energy required for the switching signals. Preferably, the entire internal energy supply of the switching device is drawn through the supply connection.
The switching device is preferably used for the purpose of switching on and off three-phase motors and also single phase alternating current motors. The motors which are connected downstream from the switching device are preferably protected against overload (short-circuit protection and/or thermal overload) by the switching device.
Insofar as the switching device is deployed in safety-critical applications, the switching device must ensure the safe shutdown of the electric motor connected downstream from the switching device.
In the case of switching devices with an emergency shutdown for the motor which is connected downstream from the switching device, one possible shutdown principle is to switch off the supply voltage to the switching device by way of a switching device (e.g. an emergency stop switching mechanism) connected in circuit in the supply line between the supply source and the supply connection. Depending on its method of functioning, switching off the supply voltage causes the electro-mechanical switches of the switching device to automatically move to the off state (i.e. the switches are opened). In this way, when the supply voltage is switched off, the energy feed to the motor through the switching device is disconnected, so that the motor is reliably switched off. Due to the disconnection of the supply source and the automatic opening of the switches, a shutdown arc forms at the switches, causing heavy wear to the switches, so that there is generally a limit to the number of such switching cycles for the switching devices.
A circuit for providing an electrical load in a protected manner is known from U.S. Pat. No. 5,953,189, which incorporates a circuit breaker and a bidirectional electronic switch connected in series. The circuit also has a shunt contactor which ensures the continuous operation of the load, and a command device, which assigns turn-on and turn-off signals to a command electrode of thyristors. As a result, the thyristors are set as to be conductive or not conductive during a start-up phase of a motor. As soon as the rated speed has been reached at the motor, the shunt contactor is closed, so that the current flow through the thyristors becomes zero. The command device is connected via a terminal to a main current path C which also supplies it with energy.
The Japanese patent application JP 2010063272 A discloses a power supply unit which is embodied to provide a constant DC voltage. The power supply unit is provided with an input-side voltmeter apparatus, which monitors a provided DC voltage. Furthermore, the power supply unit has a capacitor, which is connected to a control unit. The capacitor acts as an energy buffer during a switching operation in the power supply unit if the provided DC voltage falls below a threshold.
A chiller system is known from WO 2011/143087 A2, said chiller system incorporating a motor, which is controlled by a control unit. The motor is driven by an AC voltage via a frequency converter, which has a current converter and a first inverter. The first inverter and the current converter are connected to one another via a DC bus having a capacitor in this case. A second inverter is connected via the DC bus, via which inverter the control unit is supplied with power. Here, the control unit is connected to a frequency converter controller, which is coupled to the first inverter. If the AC voltage supply is interrupted, the inverter and the control unit in the DC bus are supplied with power via the capacitor, so that the operation of the motor is maintained for five to 15 minutes.